1. Technical Field
The present invention relates to the field of dynamic brakes for electric motors, and, more particularly, to an electronic braking circuit system and method for providing an electromotive braking force for an electric motor having no permanent magnets, for example, in an emergency situation such as a motion controller failure, or actuation of a limit switch by a driven member of a linear motor.
2. Related Art
Braking of electric motors having permanent magnets typically is accomplished by shorting the motor coils, for example, through a resistor thereby applying a large load to the electromotive force generated in the motor's coils in the presence of permanent magnets. Of course, such a solution is not possible with motors having no permanent magnets. For example, according to U.S. Pat. No. 3,832,613, a permanent magnet motor is simply controlled by a dynamic brake 162 which takes advantage of the generator action of the motor to create a reverse torque for braking. Dynamic brakes for DC motors are also described in U.S. Pat. Nos. 3,786,329 and 4,767,970.
A method and system for braking an electric motor having no permanent magnets is described in U.S. Pat. No. 5,828,195. However, the braking system according to this patent incorporates position tracking sensors, which, inter alia, significantly increases the cost of the system. In addition, for braking a linear motor the sensors have to be mounted on the moving member, which requires special fixtures and expensive flex cables. The accuracy of the location of the sensor in relation to the specific motor parts is very critical, because a small displacement from the optimum position can significantly affect the braking performance or even cause acceleration of the motor.
It can be seen from above, that there exists in the art a requirement for a system and method, of braking without using any position tracking devices, for braking an electric motor having no permanent magnets.
Thus, it is one object of the present invention to provide a sensorless braking system for utilization in braking electric motors having no permanent magnets.